Stress-Strain Behavior of FRP-Confined Concrete under Different Load Paths

Abstract

Currently, many buildings in the world have developed local failure as a result of long-term exposure to external forces. Additionally, varying degrees of damage caused by natural disasters, such as earthquakes, typhoons and mudflows, affect the normal usage of many buildings. These problems require reinforcement or structural repair of the damaged buildings. In recent years, a reinforcement material, Fiber Reinforced Polymer (FRP), has been widely used in building structure repair or reinforcement due to its ability to effectively improve the strength and ductility of building structures.

Reinforced concrete (RC) columns are major load-bearing members of building structures that bear various types of external loads. However, existing experimental studies and theoretical analyses have primarily focused on the mechanical performance of FRP-confined concrete columns under axial compression, whereas research on the mechanical properties of FRP-confined concrete columns under other loading paths are scarce. Because the stress-strain relationship is the basis for the study of mechanical performance of FRP-confined concrete columns, it is of great importance to investigate the effect of different loading paths on the stress-strain relationship of FRP-confined concrete.

Firstly, an experimental database is created by using the test data obtained from published literature for FRP-confined concrete columns with circular, square, rectangular or oval sections. Based on the equivalent constraint principle, an equivalent corner radius model for an oval is created. Based on the proposed equivalent corner radius model, an oval is equivalent to a rectangle with a corner radius. Based on the established experimental database and equivalent corner radius model, a unified compressive strength model, unified ultimate strain model and unified stress-strain relationship model for FRP-confined concrete columns with circular, square, rectangular and oval cross sections are created. Finally, the proposed model is compared to and analyzed against the existing models. The comparative analysis results show that the proposed unified stress-strain relationship model, unified compressive strength model and unified ultimate strain model for FRP-confined concrete columns with circular, square, rectangular and oval sections exhibit better forecast accuracy than the existing models.

Secondly, theoretical analysis and data verification show that the energy balance method cannot be directly applied to calculate the ultimate strain of FRP-confined concrete columns. Therefore, a modified energy balance method is proposed. Based on the modified energy balance method, a unified ultimate strain model for FRP-confined concrete columns under axial load and eccentric load is proposed. The unified ultimate strain model can calculate the ultimate strain of FRP-confined concrete columns with circular, square and rectangular sections. Compared to existing models, the proposed ultimate strain model exhibits more accurate results.

Thirdly, an experimental study on axial loading of FRP-confined concrete under various strain rates and layers of wraps is conducted. The experimental results show that the damage zone and compression strength of FRP-confined concrete cylinder are related to the strain rate. With the strain rate increases, the compressive strength and damage zone also increases. The stress-strain relationship of FRP confined concrete column under different strain rates is also investigated. Those results are combined with published test data to propose a stress-strain relationship model and compressive strength model that consider strain rate.

Finally, experimental study and theoretical analysis of the mechanical behavior of FRP-confined concrete columns under constant axial load and gradually increasing bending moment are conducted. The test results show that FRP-confined concrete columns under different loading paths exhibit different M-N interaction chart, hoop strain variation rules and axial strain variation rules. The calculation results from the stress-strain relationship model under axial and eccentric compression are significantly different from the test results. Based on the test data, a stress-strain relationship model and ultimate strain model for FRP-confined concrete under constant axial compression load and gradually increasing bending moment are proposed. The accuracy of the stress-strain relationship model is verified by using existing test data.

Date of Award	21 Jun 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yufei WU (Supervisor)